This invention pertains to the field of control systems for scale model railroad layouts, and specifically to improvements in power handling capacity of decoders used for control of elements around the layout.
Modern layout control systems allow the simultaneous control of many devices using decoder devices that are attached to or run on the tracks of model railroads. The ability to make smaller, less expensive and more reliable decoders is of great benefit, allowing the usage of the control technology in smaller railroad components and allowing greater flexibility in packaging design and installation.
The decoders derive power and control information via at least a two conductor electrical connection to the control system. This connection may be via wheel or slider pickups from the tracks, overhead catenary wires or any other conductive connection to the layout control system. The decoder analyses the encoded voltage waveform or signal conducted from this control system and, by using the information encoding rules defined for the control system, can detect and decode commands that are sent for execution or action required by the decoder.
Since the decoders may be connected in either orientation or polarity to the control system, they require an input rectifying full bridge arrangement to ensure a consistent and predictable voltage polarity can be extracted from the encoded voltage waveform when connected either way. This input rectifying full bridge carries the load current that the decoder then supplies in a switched or modulated manner, using additional power control elements, to a controlled load such as motors or lamps. The internal power control components of the decoder require an unvarying polarity inside to the decoder to operate correctly.
Additionally, the voltage encoding waveforms employed by some control systems may appear as a bipolar, or continuously alternating polarity, voltage waveform at the decoder. This then mandates the inclusion of a rectifying full bridge (also known as full wave bridge) at the input to condition the voltage waveform so it can be used by the decoder to power attached controlled loads such as motors, lamps or actuators.
Practical rectifying full bridge implementations typically include four semiconductor power diodes arranged in the full bridge rectifier configuration well known to those skilled in the art of electronic circuit design. In operation, the full bridge diode components experience a voltage drop in the forward voltage direction when conducting current. This forward voltage drop occurs while the diodes are conducting the full load current and so can represent a significant power loss. To minimize this power loss in a decoder, it is usual to use high quality and low forward voltage drop diodes, such as schottky barrier diodes. These devices represent the best conventional devices that can and have been used in prior art decoder designs.
The heat generation by the input rectifying full bridge imposes fundamental limits to the size and current control capacity of a decoder. As decoder designs strive for miniaturization the overall surface area decreases and consequently the heat dissipation capability also decreases. For a given amount of heat generation due to load current, decreased dissipation capabilities leads to increased internal temperatures and consequently lower long-term reliability. The fill bridge is the limiting device in the decoder design because the other power switching devices can take advantage of power switching devices, such as MOSFETs selected for very low losses and negligible voltage drops, at the current levels in use. Conventional rectifiers always have a minimum forward conduction voltage and losses.
To allow for a breakthrough in decoder miniaturization and increased current capacity a new and novel approach is required.
The key breakthrough is to discard prior art and to reconfigure the full bridge rectifier function with a new circuit topology hitherto unused in model railroad decoder design practices.
State of the art designs in high-energy switching power supplies, such as the design shown by Schwartz in U.S. Pat. No. 5,552,695, sometimes employ the unique conduction characteristics of metal-oxide-silicon field effect transistors, or MOSFETS, operating in their third-quadrant conduction mode. This is sometimes generically referred to as xe2x80x9csynchronous rectificationxe2x80x9d. This mode takes advantage of the MOSFET""s ability to conduct significant reverse current at a low voltage drop when the source to drain terminals are reverse biased while the gate to source terminals are forward biased or on. This technique is used to improve the efficiency of the power supply at high currents and low output voltages, since rectifier power losses are reduced and are a smaller percentage of the output voltage. Herein the term xe2x80x9cthird quadrantxe2x80x9d is taken to mean the operation of a MOSFET with drain to source terminals in reverse bias whilst the gate to source terminals are biased on.
Synchronous rectifier designs are commonly limited to half wave, series-parallel or forward-flyback rectifier configurations at the power supply inductive energy storage node or output node, since all commercial design arrangements operate with a fixed power supply output connection polarity. Here, a full bridge configuration is redundant or impractical and would have twice the components and losses on the minimally sufficient half-wave design. The ac power line input to the power supply cannot practically use a synchronous full wave bridge because suitable device ratings are unavailable at these voltage levels, or are prohibitively expensive. Also the reduction of forward voltage from, for example 0.75 Volts to about 0.3 Volts represents a negligible efficiency saving on a switched voltage of several hundred volts or more. For these reasons wholly synchronous rectifier full wave bridges have not been practically required or used to date.
MOSFETs have a parasitic or intrinsic body diode between the source and drain terminals that is off, or reverse-biased, in normal first-quadrant operation. When the MOSFET source and drain terminals are reverse biased with a zero volt gate bias, this intrinsic body diode will conduct, but this body diode operates with similar voltage drops or losses to high speed non-schottky diodes, and is unsatisfactory for efficient operations. An example of this low efficiency usage of MOSFET body diodes for rectification is the Lenz Electronics LE077XF model locomotive decoder, a circa 2000 era design. Here two six pin devices, each containing two MOSFETs with zero gate-bias, provide four independent body diodes connected conventionally as a full rectifier bridge. This design is employed to provide a full rectifier bridge in a small space, but fails to obtain the advantage of the insight or innovation of using these same devices for high efficiency third quadrant MOSFET operation.
While in reversed drain to source bias and the body diode conducting, the application of a forward, or on, instead of zero bias voltage to the MOSFET gate will induce a current carrying mode with a significantly lower voltage drop than the body diode or even schottky diodes. In operation, a low Rds(on) or high current MOSFET used as a third-quadrant rectifier can typically have losses of 25%, or less, of even the best conventional schottky rectifiers. For example, the voltage losses at load may be in the range of 0.1 Volts to 0.2 Volts for an N-channel MOSFET as a third quadrant rectifier, where a schottky rectifier would be approximately in the range of 0.55 Volts to 0.7 Volts at the same device die size and ratings in forward conduction mode.
Conventional power switching designs stringently avoid the use of P-channel MOSFETs, since the manufacture of these devices necessarily yields performances of about half of the equivalent N-channel devices. Designers go to great lengths to arrange circuit topologies to allow for N-channel devices whenever possible. For this reason, there are no prior decoder full rectifier bridge designs employing complementary N and P-channel MOSFETs in a full bridge design that employ high efficiency third quadrant techniques. The recently introduced Lenz LE010XF decoder in fact is a rare instance of the use of complementary N and P-channel devices in a full bridge, but again this design employs a zero gate bias on the MOSFETs, so is clearly not a third quadrant design.
An additional issue that good decoder designs need to consider is protection of the typical power devices used in a complementary H-bridge output to drive a DC motor.
Contemporary designs typically use two N and two P-channel MOSFETs in the well known H-bridge configuration to provide a reversible conduction path to control a DC motor in both forward and reverse directions. The installation and operation of the decoder by an unskilled user may easily result in circumstances that readily overstress or damage a decoder, most often the motor control H-bridge. The common faults are; inadvertent connection of one or both of the motor leads to a layout power connection, using a motor and mechanism with an excessive current draw, or the shorting of the motor leads due to poor wiring or insulation techniques. These connection flaws may destroy a decoder if not promptly detected in installation or operation.
Decoders may sense the motor voltages on both leads of the DC motor when the H-bridge is non-conducting to see if an incorrect or parasitic connection to another source of power is present. This represents an efficient capability and implementation.
Additionally a current sense resistor may be used in the motor circuit to see if during a H-bridge conduction phase that the motor current does not exceed a preset limit. The current sense resistor is typically bulky and can develop a excess heat at full motor load. An example of this would be the contemporary Zimo Electronics MX61 decoder that uses 0.22 ohm resistors to sense the motor load current, and also separately samples the motor back-emf for load compensation.
The on resistance of the typical H-bridge low-side N-channel MOSFETs is well characterized by device grading and manufacturer specification. In fact, it is possible to use this on resistance of these MOSFETs to sense the load current of the motor by sensing the voltage drop when ON, since one of these devices is typically always ON when the DC motor is operating in either direction. The decoder already employs voltage sensing connections to either side of the DC motor to check for motor safety isolation, as noted earlier. These same voltage samples are also used in some designs to sample the back-emf of the motor when it is coasting, to allow for load sensitive speed compensation by the decoder. Since the infrastructure for motor voltage detection is already in the design for other compelling reasons, it is a novel and useful extension to use the same voltage samples in new way so as to detect if the load current is excessive. This provides a useful and very compact design capable of giving comprehensive H-bridge protection at no extra cost and component count. The MOSFET current sense voltage is strongly influenced by die temperature, with an approximate change of +60% in voltage going from 25 to 150 degrees Celsius device temperature. This is actually beneficial since a an overheating decoder, due to poor installation and heat removal, will appropriately reach an Overcurrent decision earlier than a decoder in good heat removal conditions. In fact the die temperature is the fundamental limit to semiconductor safe operating levels, not a particular current level, so this method of Overcurrent detection has the benefit of jointly screening for adverse temperature conditions as well as absolute operating current.
Wilcox in U.S. Pat. No. 5,847,554 uses MOSFET on resistance and associated load current voltage drop to sense the correct inductive charging current and switch point for a step-down buck voltage converter. This is a completely different application of the voltage sensing effect, since this is not a decoder H-bridge and the sensed MOSFET device is not in continuous conduction. This is a completely different configuration than found in a decoder, but indicates the usefulness of current sensing by this method. Wilcox also cites overload protection by this method, but this is also different from that obtained in this invention since the Wilcox load current and converter duty cycle are modified from an inferred load current condition, because a significant inductive reactance exists between the actual MOSFETs and load and isolates this load. In the decoder H-bridge case, the load current is directly sensed with no protective or isolating impedance between the MOSFETs and load. The usage of sensed voltages is significantly different in topology and design in this invention. MOSFET on state voltages have been used prior to Wilcox in many designs for current sensing of power switches.
The value of this invention is the incorporation of this current sense method into an existing decoder design, by realizing that all the required hardware elements are in fact in place and recognizing that correct usage of the existing information provides a considerable new benefit.
Shrinking the physical size of conventional decoder designs using the prior art leads to configurations where full bridges implemented with low efficiency rectifier devices limit the available miniaturization because of thermal limitations.
Improvements in power management efficiency of full bridge rectifiers described in this invention allow creation of decoders with improved current capacity and reliability, in a smaller size.
Evaluation of voltage samples from the H-bridge motor output terminals allows the decoder to sense current overload and temperature sensing conditions with no additional hardware requirements.
All these improvements disclosed herein are best employed in a single decoder design, but may also be employed separately as required.